Information Technology Reference
In-Depth Information
Table 3-5: A string descriptor has three or more fields.
Byte
Field
Size
(bytes)
Description
0
bLength
1
Descriptor size in bytes
1
bDescriptorType
1
The constant String (03h)
2
bSTRING or
wLANGID
varies
For string descriptor zero, an array of 1 or more
Language Identifier codes. For other string
descriptors, a Unicode string.
returns status. When reading or writing blocks of data, the host identifies
the locations to read or write to by specifying a logical block address. The
USB communications don't have to know or care anything about files,
directories, or data clusters in the media.
In the bulk-only transport protocol, a successful communication has two or
three phases: command transport, data transport (not used for some com-
mands), and status transport. (Don't confuse these phases with the phases of
a USB transaction or the stages of a USB control transfer.) In the com-
mand-transport phase, the host sends a command block in a structure called
a command block wrapper (CBW). In the data-transport phase, the host or
device sends data. Some commands don't have a data-transport phase. In the
status-transport phase, the device sends status information in a structure
called a command status wrapper (CSW).
The mass-storage and SCSI specifications don't define how long a host
should wait for a device to return requested data or accept received data
before giving up. The drivers in Windows and other operating systems typi-
cally wait 20-30 seconds.
The Command Block Wrapper
The host sends the CBW to the device's bulk OUT endpoint. The CBW
contains a command block and other information about the command
(Table 3-6). The CBW is 31 bytes.
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